Nonreciprocal Metasurfaces with Epsilon-Near-Zero Materials

Nonreciprocal optics enables the asymmetric transmission of light when its sources and detectors are exchanged. A canonical example─optical isolator─enables light propagation in only one direction, similar to how electrical diodes enable unidirectional flow of electric current. Nonreciprocal optics today, unlike nonreciprocal electronics, remains bulky. Recently, nonlinear metasurfaces opened a pathway to strong optical nonreciprocity on the nanoscale. However, demonstrations to date were based on optically slow nonlinearities involving thermal effects or phase transition materials. In this work, we demonstrate a nonreciprocal metasurface with an ultrafast optical response based on indium tin oxide in its epsilon-near-zero regime. It operates in the spectral range of 1200-1300 nm with incident power densities of 40-70 GW/cm². Furthermore, the nonreciprocity of the metasurface extends to both amplitude and phase of the forward/backward transmission, opening a pathway to nonreciprocal wavefront control at the nanoscale.